Balancing Selection and Mate Choice at the Major Histocompatibility Complex

Balancing selection can maintain genetic diversity at genes such as those of the major histocompatibility complex (MHC). The highly polymorphic MHC genes make cell-surface proteins that display peptides to the immune system, thereby playing a key role in im¬mune defense. A growing number of studies have found that animals prefer mates whose MHC Class II alleles are different from their own, thereby increasing offspring fitness via heterozygote advantage. Controversially, different studies have found different patterns: animals choose mates that are either (1) as MHC-dissimilar as possible, or (2) more different than random but less different than the maximum available level of MHC-dissimilarity. We hypo¬thesize that mate choice in a given population depends on the amount of MHC diversity. In a population with only a small amount of MHC diversity, individuals should choose maximally different mates, whereas in a population with very great MHC diversity, the auto-immune costs of MHC diversity may come into play such that individuals choose mates that are ‘intermediately dissimilar’ relative to the available pool of potential mates. Don Dearborn and I will meet at NESCent to aggregate data from published studies to test a critical prediction of this hypothesis: studies that have found mate choice for maximally dissimilar MHC should have lower MHC diversity in their samples than studies that have found mate choice for intermediately dissimilar MHC.